Optically variable devices are used in a wide variety of applications, both decorative and utilitarian. Optically variable devices can be made in variety of ways to achieve a variety of effects. Examples of optically variable devices include the holograms imprinted on credit cards and authentic software documentation, color-shifting images printed on banknotes, and enhancing the surface appearance of items such as motorcycle helmets and wheel covers.
Optically variable devices can be made as film or foil that is pressed, stamped, glued, or otherwise attached to an object, and can also be made using optically variable pigments. One type of optically variable pigment is commonly called a color-shifting pigment because the apparent color of images appropriately printed with such pigments changes as the angle of view and/or illumination is tilted. A common example is the “20” printed with color-shifting pigment in the lower right-hand corner of a U.S. twenty-dollar bill, which serves as an anti-counterfeiting device.
Some anti-counterfeiting devices are covert, while others are intended to be noticed. Flakes having covert features therein, such as indicia, gratings, and holographic features, can be used in addition to overt features. Furthermore flakes with can be used. Unfortunately, some optically variable devices that are intended to be noticed are not widely known because the optically variable aspect of the device is not sufficiently dramatic. For example, the color shift of an image printed with color-shifting pigment might not be noticed under uniform fluorescent ceiling lights, but more noticeable in direct sunlight or under single-point illumination. This can make it easier for a counterfeiter to pass counterfeit notes without the optically variable feature because the recipient might not be aware of the optically variable feature, or because the counterfeit note might look substantially similar to the authentic note under certain conditions.
Optically variable devices can also be made with magnetic pigments that are aligned with a magnetic field after applying the pigment (typically in a carrier such as an ink vehicle or a paint vehicle) to a surface. However, painting with magnetic pigments has been used mostly for decorative purposes. For example, use of magnetic pigments has been described to produce painted cover wheels having a decorative feature that appears as a three-dimensional shape. A pattern was formed on the painted product by applying a magnetic field to the product while the paint medium still was in a liquid state. The paint medium had dispersed magnetic non-spherical particles that aligned along the magnetic field lines. The field had two regions. The first region contained lines of a magnetic force that were oriented parallel to the surface and arranged in a shape of a desired pattern. The second region contained lines that were non-parallel to the surface of the painted product and arranged around the pattern. To form the pattern, permanent magnets or electromagnets with the shape corresponding to the shape of desired pattern were located underneath the painted product to orient in the magnetic field non-spherical magnetic particles dispersed in the paint while the paint was still wet. When the paint dried, the pattern was visible on the surface of the painted product as the light rays incident on the paint layer were influenced differently by the oriented magnetic particles.
Similarly, a process for producing of a pattern of flaked magnetic particles in fluoropolymer matrix has been described. After coating a product with a composition in liquid form, a magnet with desirable shape was placed on the underside of the substrate. Magnetic flakes dispersed in a liquid organic medium orient themselves parallel to the magnetic field lines, tilting from the original planar orientation. This tilt varied from perpendicular to the surface of a substrate to the original orientation, which included flakes essentially parallel to the surface of the product. The planar oriented flakes reflected incident light back to the viewer, while the reoriented flakes did not, providing the appearance of a three dimensional pattern in the coating. It is desirable to create more noticeable optically variable security features on financial documents and other products and to provide features that are difficult for counterfeiters to copy.
It is also desirable to create features which add to the realism of printed images made with inks and paints having alignable flakes therein, especially printed images of objects and more particularly recognizable three dimensional objects.
Heretofore, in patent application PCT/US2003/020665 the inventor of the present application has described the “rolling-bar” and the “flip-flop” images which provide kinematic features, that is features which provide the optical illusion of movement, to images comprised of magnetically alignable pigment flakes wherein the flakes are aligned in a particular manner.
It has been discovered that providing a rolling bar used as a fill within an outline of a curved recognizable object, particularly a smooth curved recognizable object such as a bell, a shield, container, or a soccer ball provides striking effects that reach beyond a rolling bar moving back and forth on a rectangular sheet. The bar while providing realistic dynamic shading to an image of an object not only appears to move across the image but also appears to grow and shrink or expand and contract with this movement within the closed region in which it is contained. In some instances where the size or area of the bar doesn't vary, for example wherein it is used a as a partial fill within an image between two conforming curved lines that move together with a space between, filled by the bar, the bar appears to move across the image while simultaneously moving up and down. Thus, a highly desired optical effect is provided by using the rolling bar inside a non rectangular outlined closed shape of an object, wherein the area of the rolling bar changes as the bar moves across the image, and, or wherein the bar appears to move horizontally and vertically simultaneously as the image is tilted or the light source upon the image is varied. Additionally, if the bar is designed to be of a suitable size and radius of curvature, it can be used as a dynamic, moving, shrinking or expanding shading element in the image, providing exceptional realism. It has also been found, that the rolling bar appears to have a most profound effect when it appears to mimic moving shading on an image of a real object that is capable or producing a shadow when light is incident upon it. In these important applications, it is preferred that the radius of curvature of the flakes forming the rolling bar be within a range of values wherein the image of the real-object it is applied to, appears to be correctly curved so as to appear realistic.
Patent Publication EP 710508A1 to Richter et al. (hereinafter “Richter”) discloses methods for providing three dimensional effects by drawing with magnetic tips. Richter describes three dimensional effects achieved by aligning magnetically active pigments in a spatially-varying magnetic field. Richter uses standard pigments (barium ferrite, strontium ferrite, samarium/cobalt, Al/Co/Ni alloys, and metal oxides made by sintering and quick quenching, none of which are composed of optical thin film stacks. Rather, the particles are of the hard magnetic type. Richter uses electromagnetic pole pieces either on top of the coating or on both sides of the coating. However, Richter uses a moving system and requires “drawing” of the image. The “drawing” method provides only limited optical effects. In particular, the “rolling-bar” and the “flip-flop” images can not be formed using this method.
The aforedescribed kinematic features, such as the “rolling-bar” and the “flip-flop” images, as well as images appearing to be 3-dimensional curved objects as a soccer ball, rely on particular, intrinsic flake patterns. By way of example, two parts of a “flip-flop” image should be clearly separated and a blurred border would downgrade the image quality. In order to form such intrinsic patterns, the high precision alignment of the flakes is required.
A method of painting an object with a paint containing magnetic flakes includes placing a magnet under or above the object's surface, painting the object using a spray gun, and leaving the object in place until the paint solvent evaporates. This method, as well as “drawing”, takes time and is not conducive to production type processes.
The optically illusive images with kinematic features, such as the “rolling-bar” and the “flip-flop” images, as well as images appearing to be 3-dimensional curved objects like, provide highly visible security features. Such features attract a person's attention, are easy to verify and difficult to forge, thus they are used more extensively over time in different applications, such as currency, documents, packaging.
Mass production requires high-speed methods of manufacturing of such images while providing high precision alignment of the flakes therein.
Accordingly, an object of the present invention is to provide a method and apparatus for aligning of magnetic flakes with a high degree of precision performed at a speed suitable for mass production.